1. Field of the Invention
The present invention relates to an infrared detecting apparatus having a thermal isolation structure, and to an infrared imaging apparatus using the infrared detecting apparatus.
2. Description of the Related Art
A basic and representative example of a thermal isolation structure, which is most frequently used in practice for a thermal infrared solid state imaging device, is disclosed in a report by R. A. Wood (Technical Digest of International Electron Devices Meeting, pp. 175-177 (1993)), the entire contents of which are incorporated herein by reference.
As shown in FIG. 4, a diaphragm 30, which is a photodetector element, is supported with two supporting legs 21 in a way that the diaphragm 30 is suspended in the air above the silicon substrate in which a transistor 22 is formed. The diaphragm 30, the supporting legs 21 and the transistor 22 form an infrared detecting apparatus. Each of the diaphragm 30 and the supporting legs 21 is made of silicon nitride of 0.5 μm thick. A temperature-sensitive resistor is disposed inside the diaphragm 30. The temperature-sensitive resistor is made of a thin polycrystalline semiconductor film of approximately 1000 Å (or 100 nm) thick.
A plurality of infrared detecting apparatuses are formed in the silicon substrate, and each of the infrared detecting apparatuses is used for one pixel. The silicon substrate is provided with a processing circuit which processes image signals outputted from all pixels. Each pixel is provided with one of the transistors 22 which perform switching. A switching signal is fed to a base 24 of each of the transistors 22 by metal wiring 25.
One of the supporting legs 21 extends from a part where an emitter 23 of the transistor 22 is formed. The other supporting leg 21 extends from metal wiring 26 orthogonal to the metal wiring 25. The temperature-sensitive resistor in the diaphragm 30 is connected to the emitter 23 and the metal wiring 26 by thin-film metal wirings provided to the two supporting legs 21. Accordingly, the supporting legs 21 are each provided with one thin-film metal wiring.
If only one supporting leg is used to support the diaphragm, the number of supporting legs is the minimum. This construction is suggested in Japanese Patent Translation Publication No. 2004-527731 (see claim 1 of this publication).
In order to improve sensitivity of an infrared imaging apparatus, it is often the case where the amount of heat flowing out of the supporting leg is reduced. The inventor has compared the amount of outflowing heat in the case where only one supporting leg supports the diaphragm, with the amount of outflowing heat in the case where two supporting legs support the diaphragm. The result of this comparison has shown that, in principle, supporting with two supporting legs makes it possible to reduce the amount of outflowing heat as compared with supporting with only one supporting leg (see FIGS. 3A to 3D).
Where a magnitude of moment is a product of a force which presses the diaphragm downward when gravity acts on the diaphragm, and a distance between the base of the supporting leg and the center of gravity of the diaphragm, it is considered that the magnitude of moment can be used as an index of strength which represents how strongly the supporting leg is deformed with the weight of the diaphragm. A cross-sectional area of the supporting leg required for suppressing deformation of the supporting leg is small when the magnitude of moment applied to the supporting leg is small. The amount of heat flowing out of the supporting leg decreases when the cross-sectional area of the supporting leg is made small. Thus, a smaller magnitude of moment makes it possible to reduce the amount of outflowing heat.
FIG. 3A shows an infrared detecting apparatus (of a one-legged type) in which only one supporting leg supports a diaphragm. In FIG. 3A, m denotes the mass of a diaphragm 40, g denotes gravitational acceleration, and L denotes a distance between the base of a supporting leg 19 and the center of gravity of the diaphragm 40. The magnitude M1 of moment is obtained by Expression (1).M1=mgL  (1)
FIG. 3B shows an infrared detecting apparatus (of a two-legged type) in which two supporting legs support the diaphragm. In FIG. 3B, the diaphragm 40, of which mass is indicated by m, is supported with two supporting legs 20. The distance between the base of each supporting leg 20 and the center of gravity of the diaphragm 40 is indicated by L as is the case with one-legged type. The magnitude of moment which each supporting leg 20 needs to withstand is indicated by M2.M2=M1/2=mgL/2  (2)When Expression (2) is satisfied, it can be considered that the lower limit of the amount of outflowing heat in the one-legged type is equal to that in the two-legged type. The reason is that the cross-sectional area of the supporting leg 19 of the one-legged type required for suppressing deformation thereof due to the moment is equal to the sum of the cross-sectional areas of the respective supporting legs 20 of the two-legged type required for suppressing deformation thereof due to the moment.M2>mgL/2  (3)
When Expression (3) is satisfied, the amount of outflowing heat can be reduced more in the one-legged type than in the two-legged type.M2<mgL/2  (4)
When Expression (4) is satisfied, the amount of outflowing heat can be reduced more in the two-legged type than in the one-legged type.
To clarify which of Expressions (2) to (4) is satisfied, magnitude M2′ of moment applied to each of supporting legs 20 in a case where two supporting legs 20 are used to respectively support two equal parts (see FIG. 3C) into which the diaphragm 40 of the two-legged type is cut is compared to each of the magnitudes M1 and M2 of moments in the case of the one-legged and two-legged types.
First, the magnitude M2′ of moment in the case shown in FIG. 3C is compared to the magnitude M1 of moment in the case of the one-legged type. In the case of FIG. 3C, gravity acting on each of diaphragms 40a is mg/2. The center of gravity of each diaphragm 40a is located closer to the base of the supporting leg 20 supporting the diaphragm 40a than the center of gravity of the diaphragm 40 of the two-legged type (see FIG. 3B). In other words, a distance L′ between each of the bases of the supporting legs 20 and the center of gravity of the diaphragm 40a supported with the supporting leg 20 is shorter than the distance L of the one-legged type, between base of the supporting leg 19 and the center of gravity of the diaphragm 40 supported with the supporting leg 19. Accordingly, the relationship between the magnitude M2′ of moment applied to each supporting leg 20 in the case shown in FIG. 3C and the magnitude M1 of moment applied to the supporting leg 19 of the one-legged type can be expressed by Expression (5) as given below.M2′=mgL/2<mgL/2=M1/2  (5)Expression (5) indicates that the magnitude of moment, which each of the supporting legs 20 shown in FIG. 3C needs to withstand, is less than one half of the magnitude M1 of moment applied to the supporting leg 19 of the one-legged type.
Next, the magnitude M2′ of moment in the case shown in FIG. 3C is compared to the magnitude M2 of moment in the case of the two-legged type when moment is applied to each of the supporting legs 20 to deform. As shown in FIG. 3D, a reaction force T occurs when each supporting leg 20 of the two-legged type is deformed under the action of the applied moment. The reaction force T produces a restoring force which acts to suppress deformation in the supporting leg. Before the occurrence of the deformation, it can be considered that the magnitude M2 of moment applied to each supporting leg 20 of the two-legged type is equal to the magnitude M2′ of moment applied to each supporting leg 20 in the case shown in FIG. 3C. The moment applied to each supporting leg 20 of the two-legged type is reduced by the restoring force, and thereby the magnitude of moment which each supporting leg 20 of the two-legged type needs to withstand decreases correspondingly. Accordingly, the relationship between the magnitude M2′ of moment applied to each supporting leg 20 in the case shown in FIG. 3C and the magnitude M2 of moment applied to each supporting leg 20 of the two-legged type can be expressed by Expression (6) as given below.M2<M2′  (6)
According to Expressions (5) and (6), a relationship among the magnitude M1 of moment applied to the supporting leg 19 of the one-legged type, the magnitude M2 of moment applied to each supporting leg 20 of the two-legged type, and the magnitude M2′ of moment applied to each supporting leg 20 in the case shown in FIG. 3C can be expressed by Expression (7) as given below.M2<M2′<M1/2=mgL/2  (7)Expression (7) indicates that Expression (4) is satisfied. Accordingly, it can be concluded that it is possible to reduce the amount of outflowing heat more in the two-legged type than in the one-legged type.
Besides the deformation in the supporting leg due to the weight of the diaphragm, another problem arises. The problem is that a microstructure including the supporting leg and the diaphragm is deformed under residual stress during the manufacture of the infrared imaging apparatus. In a case of the one-legged type, the supporting leg deforms in a way that the greatest displacement occurs in a portion of the diaphragm positioned farthest from the base of the supporting leg. Deformation cannot be suppressed in the one-legged type because this type is not provided with means which reduces the displacement to position the diaphragm appropriately. Deformation can be suppressed in the two-legged type more than in the one-legged type because the two-legged type allows the portion of the diaphragm positioned farthest from the base of one supporting leg to be supported with the other supporting leg.
As mentioned above, the amount of outflowing heat can be reduced more in the two legged type than in the one-legged type. The supporting legs of the two-legged type, as shown in FIG. 4, are each provided with one thin-film metal wiring. A smaller width of the metal wirings leads to higher resistance of the metal wirings. The higher resistance of the metal wirings then leads to a lower rate of change in the resistance (change in temperature) of the temperature-sensitive resistor, to the sum of the resistance of the temperature-sensitive resistor operating at room temperature and the resistance of the metal wirings. For this reason, sensitivity of the temperature-sensitive resistor is lowered. Due to a trade-off relationship between the width of the supporting leg and the resistance of the metal wiring, the width of the supporting leg is relatively large so as to cause the resistance of the metal wiring to be relatively low. As a result, the amount of outflowing heat is large in the two-legged type, as shown in FIG. 4.